Immunoassays, mainly in the form of enzyme-linked immunosorbent assays (ELISAs), have been widely used in the area of clinical diagnostic analysis (Gosling, Clin. Chem., 36:1408-1427 (1990)). These methods offer high specificity, sensitivity, and ease of operation over other standard laboratory procedures. However, some of the disadvantages of the ELISA format which necessitate further improvement on the methodology include the lengthy time required for antigen-antibody reaction, reagent additions, enzymatic conversion of substrate, and several washing steps between the various operations.
As an alternative to the use of enzymes, liposomes are of interest as detectable labels in immunoassays because of their potential for immediate signal amplification. Liposomes are spherical vesicles in which an aqueous volume is enclosed by a bilayer membrane composed of phospholipid molecules (New, Liposomes: A Practical Approach, IRL Press, Oxford (1990)). Previous studies (Plant et al., Anal. Biochem., 176:420-426 (1989); Durst et al., In: GBF Monograph Series, Schmid, Ed., VCH, Weinheim, FRG, vol. 14, pp. 181-190 (1990) have demonstrated the advantages of liposome-encapsulated dye over enzymatically produced color in the enhancement of signals in competitive immunoassays. The capillary migration or lateral flow assay avoids separation and washing steps and long incubation times and yet attains sensitivity and specificity comparable to ELISAs. Nevertheless, the methodologies (Siebert et al., Anal. Chim. Acta, 282:297-305 (1993); Roberts et al., Anal. Chem., 67:482-491 (1995); Siebert et al., Anal. Chim. Acta, 311 :309-318 (1995); Reeves et al., Trends Anal. Chem., 14:351-355 (1995); Rule et al., Clin. Chem., 42:1206-1209 (1996)) involve operations and solutions that make the handling of the sample and reagents susceptible to errors and more difficult to use for untrained personnel.
The driving force behind the formation of liposomes is hydration, so that when water is removed from lipid membranes, a shift in the phase transition occurs and a phase separation of lipids can take place resulting in aggregation and fusion of the liposomes, thereby losing the barrier function of the membrane (Lasiv, Biochim. Biophys. Acta, 692:501-502 (1982); Mobley et al., J. Control Release, 31:73-87 (1994); Crowe et al., cryobiology, 19:317-328 (1982); Lin et al., Stud. Biophys., 127:.99-104 (1988); Crowe et al., J. Bioenerg. Biomembr., 21:77-92 (1989)).
The present invention is directed to overcoming the above-noted deficiencies in the prior art.